The efficiency of a MOSFET logic circuit depends upon its current driving capability. The current drive in turn depends upon the device threshold voltage which is a function of the source to substrate voltage difference. Because the source voltage varies during certain circuit applications such as ungrounded sources, the source to substrate voltage varies. As a result, the threshold voltage varies so that the current drive capability of the device is modified. The problem is to make the threshold less sensitive to changes in the source to substrate voltage of the device. The rate of change of threshold voltage with respect to the source to substrate voltage is commonly referred to as the substrate sensitivity of the device. Substrate sensitivity is a function of factors like oxide thickness, doping in the background, dielectric constant, etc. Thus an objective has been to reduce the variations in threshold voltage during device operation by reducing the substrate sensitivity resulting in improved current drive capability.
Various prior art attempts have been made to improve substrate sensitivity. For example, one alternative considered was to use high resistivity substrates by uniformally altering substrate dopant concentration because it was also thought to give better capacitance. However, when using high resistivity substrates, inherently problems of density resulting from short channel effects, punch-through, etc. are created. When the over all background conductivity is reduced, inversion may result in the field regions causing circuit malfunction. Although there are some gains attained by using high resistivity substrates, a significantly large amount of the gain is offset by the inherent problems.
Another attempt to achieve lower substrate sensitivity is substrate isolation by separating the substrate for each device. The cost of fabrication becomes excessive because of complex processes needed such as double diffusion and doping two different regions. Also, there is a loss in density inherent in isolating any device.
It is known to double ion-implant in the channel region of an FET device to move an implanted depletion mode load type junction toward the surface of the channel to solve the problem of a depletion mode device which cannot otherwise be controlled or turned-off. This is accomplished by double ion-implanting materials of opposite type to yield an abrupt junction. However, substrate sensitivity reduction does not necessarily occur simply by ion-implantation.